This invention relates to cold emission semiconductor devices.
There are known cold electron emission semiconductor devices, such as cathodes, comprising p-n junctions with homogeneous forbidden band gaps, such as silicon (Si), gallium arsenide (GaAs) and gallium-arsenic-phosphorus (Ga(AsP)). In these prior devices work function is decreased by cleaning the surfaces and activating with cesium or cesium and oxygen. Thus, the prior devices are made so that the electrons passing through the junctions are emitted into vacuum from the surfaces. It has also been previously proposed to use, in such devices, n-(AlGa)As-- p-GaAs different type or sometimes known also as heterogeneous junctions wherein the effective forbidden band gap of the n-layer is made greater than that of the p-layer, in order to inject electrons from the n-layer into the p-layer with good efficiency. That is, when constructing the p-n junction in a semiconductor device having a homogeneous forbidden band gap, such as silicon, the injection amount of the holes from the p-type region to the n-type region increases considerably as a result of raising the impurity concentration in the p-type region to lower the work function of the surface. Accordingly, the efficiency of injection of electrons to the p-type region is markedly lowered and the cold emission efficiency is reduced.
With silicon, particularly, since the forbidden band gap is as small as 1.107 eV, considerable limitation occurs in the manufacture of electron emission surfaces of zero to negative electron affinity. Consequently, the heterojunctions, such as mentioned above, were proposed in order to inject electrons into the p-type region with good efficiency. In such apparatus, however, there is high probability that the electrons will recombine because the gallium arsenide to which the electrons are injected, is a direct transition type semiconductor. Consequently, before the injected electrons reach the surface, a considerable amount of them will be lost by recombination. In order to decrease this recombination, it has been priorly considered to make the p-type layer thinner than the diffusion length of the electrons. However, since there is need to furnish ohmic contacts so that the electron emission will not be hindered in the p-type layer, if the p-type layer is made thinner, the resistance in the latitudinal direction is increased. Also, since the (AlGa)As layer has poor thermal conduction, the injection density of the electrons cannot be raised, so a point cathode cannot be formed.
Thus, there are numerous disadvantages and deficiencies in prior art devices, which are desirous of reduction or elimination.